Reduction of viscosity of cellulose nitrate



'PatcntcdMay's, 1936 UNITED STATES PATENT OFFICE REDUCTION OF OF CELLULOSE- NITRATE No Drawing. Application October 22, 1931,

Serial No. 570,503

14 Claims.

This invention relates to the treatment of cellulose nitrate and more particularly to the treatment of cellulose nitrate in the undissolved state of reduced viscosity adopted in commercial production, nor has it been of practical value for the production of high grade low viscosity nitrocellulose for use in lacquers, films, etc., because earlier processes did not utilize the preferred method of bleaching described hereinafter. Due to the ammonia bodies not being competely removed, a color equal to that of the best commercial nitrocotton was not obtained and a further reduction in viscosity occurred in the final product.

The present invention has for an object the provision of a simple, practical process of treating cellulose nitrate to give a stable product of low viscosity and'the desired whiteness without degradation of the cellulose nitrate and loss of tensile strength in resulting films. A further object is to provide a process that can be carried out on a commercial scale and that will give high yields.

The treatment of cellulose nitrate according to this invention is effected by subjecting cellulose nitrate, either bleached or unbleached, in the undissolved state to the action of an ammonia solution until the desired reduction in viscosity is obtained, washing the cellulose nitrate, and then bleaching the cellulose nitrate with an aqueous solution containing available chlorine, removing the solution, boiling the product with water to which an anti-chlor may be added, if desired, and repeating the last two steps, if necessary, until a product of-satisfactory color is obtained.

The particular type of cellulose nitrate is not important in the present invention, although the more carefully the cotton linters or other sources of cellulose are purified and nitrated, the better the final product will be. However, the treatment of the cellulose material to form the cellulose nitrate to be used in this process need not differ from the treatment to produce cellulose. nitrate for any other subsequent viscosity reducing process. Usually, the cellulose nitrate will be the ordinary nitro-cotton made from cotton linters. If desired, the cotton linters of cellulose nitrate produced therefrom may be bleached before the treatment process of the present invention commences, as that will reduce the amount of the subsequent bleaching necessary. But, as the cellulose nitrate will have to be bleached after the ammonia treatment step, bleaching prior thereto is not essential.

Although it is preferred to use the ordinary non-colloided nitro-cotton made from cotton linters, the process is not limited to this type of cellulose nitrate, since nitrated paper, nitrated bagass'e, nitrated wood pulp, nitrated dense cellulose and other forms of cellulose nitrate may be used. Also colloided cellulose nitrate may be used, such as precipitated smokeless powder which, for example, may be prepared by dissolving smokeless powder in acetone and precipitating by Pouring the solution in water to get the cellulose nitrate in finely divided form.

The first step of the present treatment comprises placing the cellulose nitrate in an open tub or other suitable apparatus, preferably equipped with recirculating means and a heating coil, and covering the cellulose nitrate with water con taining ammonia equal to about 0.5% to 11.2% by weight of the dry cellulose nitrate. The rate of viscosity reduction is dependent upon the temperature. The temperature is maintained be tween 70 and 100 C., preferably, for anywhere from 2 to 24 hours, depending upon the final vis-' ccsity desired. Lower temperatures may be used but longer time cycles are then required. The reduction in viscosity caused by the subsequent bleaching steps must be taken into consideration and the ammonia treatment stopped when the viscosity is reduced sufiiciently since, during the bleaching treatment, the reduction in viscosity will continue to a lessened extent until all of the ammonia has been removed from the fibers. Samples of tlie cellulose nitrate can be taken during the treatment to determine the extent of the reduction of viscosity at various stages. Other solvent mediums may be used to carry the ammonia, such as alcohol, but water is preferred.

The heating is preferably accomplished by means of a heating coil placed between the bottom of the tub and a perforated false bottom which prevents the cellulose nitrate from contacting with the heating coil, but allows heating of the aqueous liquor. It is preferred to keep the ammonia solution recirculating during the treatment, but this is not essential. When the desired reduction of viscosity has been reached, the ammonia solution is drained off and the cellulose nitrate washed thoroughly with water.

The cellulose nitrate is now bleached conveniently, without removal from the tub, by an aqueous chlorine solution or equivalent chlorine bleaching solution, such as acidified calcium hypochlorite. Regardless of the solution used, the available chlorine content of the solution should not greatly exceed 15.0%, and preferably should not exceed 3.0% by weight of the nitrocellulose, and may be as low as 0.375%. After bleaching, the cellulose nitrate is boiled with water to thoroughly remove the chlorine. An anti-chlor, such as oxalic acid, may be added to the water, if desired. If the product is not of sufiicient whiteness, or the ammonia is not completely removed from the cellulose nitrate, the bleaching step should be repeated, keeping the available chlorine within the above limits, until the color is satisfactory. If the last bleaching step is mild, it is sufficient to thoroughly wash the product with hot water, rather than boiling the product with water.

According to the present process, the viscosity reduction is attained by chemical means and the viscosity reducing agent is removed by chemical reaction during the. bleaching treatment. Heretofore this had not been done in processes employing ammonia as the reducing agent and, as a result, the viscosity of the final product was not stable and, furthermore, the presence of the ammonia and/or ammonia reaction products inhibited the action of the bleaching agent so that a satisfactory bleach was not possible. Removal of the ammonia and/or ammonia reaction products by washing cannot be carried to completion, whereas by the herein described chlorine bleaching treatment and boiling, a complete chemical removal of the ammonia and ammonia reaction products is effected and consequently a satisfactory bleach is attained and the viscosity oi? the final product is stable.

The end products of the reaction between the chlorine used in the bleaching treatment and the ammonia and/or ammonia reaction products absorbed in the cellulose nitrate are N2 and HCl. Presumably they are formed as indicated in the following table for the reaction between ammonia and chlorine:

By the use of nitrous acid in place of chlorine the ammonia and/or ammonia reaction products can likewise be removed from the cellulose nitrate and the color resulting from the ammonia treatment destroyed. This reagent does not, however, remove the color which is inherent in the raw cotton lint or other source of cellulose, so that a subsequent and additional bleaching treatment with standard bleaching agents is necessary.

To illustrate the invention, the following specific examples are given in which the viscosity of the nitrocellulose is given in seconds as determined by the method specified in the Tentative Specifications and Tests for Soluble Nitrocellulose" under designation 301--29T, American Society for .Testing Materials. The 135 stability test" referred to is described in Designation D301-30T American Society for Testing Mate- Example 1;Nitro-cotton having a nitrogen content of 12.42% and a viscosity of 106 seconds in 12% solution was drowned in water, boiled for four hours, and then washed with hot water. The nitro-cotton was then covered with water containing ammonia equal to 2% by weight of the dry nitro-cotton and held at 70 C. for 24 hours. It was then washed twice with cold water and bleached with three acidified hypochlorite solutions successively, containing 2.25%, 1.5%, and .75% available chlorine, respectively. After each of the first two bleaches, the hypochlorite solution was replaced with fresh water and the slurry boiled for two hours. After the last bleach, which was relatively mild, the product was washed four times with water and then dehydrated.

The final product had a nitrogen content of 12.20%; stability of 30 minutes by the 135 test; viscosity of 13.8 seconds in 20% solution; and color of on the Hazen scale. The tensile strength and flexibility of film prepared from the product were equal to those of the best commercial low viscosity nitro-cotton, as was also the, length of fiber.

Example 2.Nitro-cotton having a nitrogen -content of 11.89% and viscosity of 14.8 seconds in 12% solution was treated with water containing ammonia equal by weight to 2% of the nitrocotton for 17%; hours at 70 C. It was then washed and bleached successively three times with acidified hypochlorite solution containing available chlorine equal by weight to 2.25%, 1.50%, and 0.75% of the nitro-cotton, respectively. After each of the first two bleaches, the bleaching liquor was drained, fresh water added, and the slurry boiled for two hours. After the last bleach, the material was thoroughly washed with hot water and dried.

The nitro-cotton after this treatment had a nitrogen content of 11.63%, stability of 30 minutes by the test, viscosity of 1.5 seconds in 20% solution, and a color of 200 on the Hazen scale.

Example 3.-Nitro-cotton having a nitrogen content of 12.07% and a viscosity of 644 seconds in 12% solution was boiled in water for four hours and washed with water twice. The nitrocotton was then covered with water containing ammonia equal to 2% of the dry nitro-cotton and kept at 70 C. for six hours. It was then washed twice with water and bleached with three solutions of chlorine water containing chlorine equal to 2.25%, 0.375%, and 0.375%, respectively, of the weight of the dry nitro-cotton. After the first and second bleaches, the material was boiled in water for two hours and after the final bleach it was washed six times with hot water. It was then dehydrated in the usual manner.

The nitrogen content of the product was 12.00%, stability, 40 minutes by the 135 testj viscosity, 4 seconds in 12% solution; and color, 200 on the Hazen scale. The physical properties of a film made from this product were equal to that of films made of the best grade of nitrocotton of similar viscosity. The length of fiber was equal to that of the best commercial nitrocotton of equal viscosity.

Example 4.-A charge of second cut lint was purified by boiling with 16 times its weight of 1% caustic soda solution for two hours at atmospheric pressure. The caustic and dissolved impurities were removed by washing and the cotton dried according to usual practice. It was then nitrated under standard conditions at 35 C. to give a product with a nitrogen content of 12.30% and very high viscosity. Aiter boiling in aqueous slurry for 4 hours and washing twice, it was covered with water containing ammonia equal in weight to 2% of the nitro-cotton and held at 80 C. for 18% hours. After being washed twice, it was bleached with chlorine water containing chlorine equal in weight to 0.75% of the nitrocotton and then washed 3 times. It was then dehydrated in the usual manner.

The product had a nitrogen content of 11.98% and stability, by the 135 test of 50 minutes. Its viscosity in solution was 20.4 seconds and color on the Hazen' scale above 500. The fiber was exceptionally good for nitro-cotton of so low a viscosity.

Example 5.A commercial purified cotton was nitrated under standard conditions to obtain a nitrogen content 01' 11.25% and viscosity in 12% solution of about 333 seconds and stabilized according to usual practice. This material was covered with water containing ammonia equal in weight to 2.4 72, of the weight of the nitro-cotton and held at 75-80 C. for 24 hours. It was then washed twice and bleached successively with acidified hypochlorite containing available chlorine equivalent to 2.25% and 1.50% of the weight of the nitro-cotton. After each bleach, the reagent was drained and the fresh slurry boiled for two hours to restore stability; the treatment was finished with 3 hot water washes and dehydration in the usual manner.

The product had a nitrogen content of 11.14% and stability, by the 135 test of 35 minutes. The viscosity of a 20% solution was 8 seconds and color by the Hazen scale, above 500. The tensile strength of a film prepared from it, and its flexibility, as well as the length of fiber, were equal to best commercial low viscosity nitro-cotton.

Example, 6.--A portion of nitro-cotton with a nitrogen content lying between 11.4 and 11.6% and a viscosity of 81 seconds in 12% solution was treated with water containing ammonia equal in weight to 0.5% of the weight of the nitro-cotton for 6 hours at 75 C. It was then washed twice and bleached with acidified hypochlorite containing available chlorine equal to 375% of the weight of the nitro-cotton, after which it was boiled with water for 10 hours, washed 5 times with water and dehydrated in the usual manner. The nitrogen content was 11.33% and stability, by the 135 test minutes. Its viscosity in 12% solution was 12 seconds and color on the Hazen scale, 100. The tensile strength of a film made from it, and its fiexibility, were equal to pyroxylin nitro-cotton of similar viscosity prepared by usual methods.

As can be seen by the above examples, through regulation of the ammonia treatment and bleaching, cellulose nitrate of practically any viscosity lower than that obtained in the ordinary process of nitrating to obtain high viscosity cellulose nitrate can be obtained without degradation of the nitrocellulose or loss of advantageous properties.

An advantage of the present process of treating cellulose nitrate is that the use of high pressures in complicated apparatus now generally used, is eliminated. The process is adapted to be completely and easily controlled and suitable for commercial scale production. An excellent grade of cellulose nitrate of low viscosity can be obtained having the whiteness and properties required in the strictest specifications for lacquer and film-type nitrocellulose. The discoloration oi the cellulose nitrate is removed without loss prising subjecting the cellulose nitrate in the undissolved state to the action of an ammonia solution to reduce the viscosity, treating same with an aqueous solution containing available chlorine, removing said solution, and boiling the cellulose nitrate in water, and repeating the last three steps.

2. Process of treating cellulose nitrate com-' prising subjecting the cellulose nitrate in the undissolved state to the action 0! an aqueous ammonia solution to reduce the viscosity, treating same with an aqueous solution containing available chlorine, removing said solution, and boiling the cellulose nitrate in water, and repeating the last three steps.

3. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an ammonia solution to reduce the viscosity, washing the cellulose nitrate with water, treating same with an aqueous solution containing available chlorine, removing said solution, and boiling the cellulose nitrate in water, and repeating the last three steps.

4. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous solution containing from 0.5 to 11.2% of ammonia by weight of the dry .cellulose nitrate to r'eduoe the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, and boiling the cellulose nitrate in water, and repeating the last three steps.

5. Process of treating cellulose nitrate comprisingsubjecting the cellulose nitrate in the undissolved state to the action of an aqueous solution containing from 0.5 to 11.2% of ammonia by weight of the dry cellulose nitrate to reduce the viscosity, washing the cellulose nitrate with water, treating same with an aqueous solution containing not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, boiling the cellulose nitrate in water, and repeating said last three steps to further remove color and free the cellulose nitrate of ammonia.

6. Process oi. treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an ammonia solution to reduce the viscosity, treating same with an aqueous solution containing available chlorine, removing said solution, and washing the cellulose nitrate with water, and repeating the last three steps.

7. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous solution containing from 0.5 to 11.2% of ammonia by weight of the dry cellulose nitrate to reduce the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, and washing the cellulose nitrate with water. and repeating the last three steps.

8. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous solution containing from 0.5-11.2% of ammonia. by weight of the dry. cellulose nitrate at a temperature of 70-100 C. to reduce the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing not more than 3% available chlorine based on the weight of the dry cellulose nitrate, removing the said solution and washing the cellulose nitrate and repeating the last three steps.

9. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous ammonia solution to reduce the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, washing the cellulose nitrate with water, and repeating said last three steps to further remove color and free the cellulose nitrate of ammonia.

10. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous ammonia solution to reduce the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing .not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, boiling the cellulose nitrate in water, and repeating said last three steps to further remove color and tree the cellulose nitrate of ammonia.

11. Process of treating cellulose nitrate comprising subjecting the cellulose-nitrate in the undissolved state to the action an aqueous ammonia solution to reduce the viscosity, washing the cellulose nitrate with water, treating with an aqueous solution containing not more than 3% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, boiling the cellulose nitrate in water, and repeating said last three steps to further remove color and free the cellulose nitrate oi! ammonia.

12. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the un dissolved state to the action oi! an aqueous solution containing from 0.5-11.2% of ammonia by weight of the dry cellulose nitrate at a temmrature of 70-100 C. for 2-24 hours, washing the cellulose nitrate with water, treating same with an aqueous solution containing not more than 15% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, boiling the cellulose nitrate in water, and repeating said last three steps to further remove color and free the cellulose nitrate of ammonia.

13. Process oi! treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an aqueous solution containing from 0.5-11.2% of ammonia by weight of the dry cellulose nitrate at a temperature of 70-100 C. for 2-24 hours, washing the cellulose nitrate with water, treating same with an aqueous solution containing not more than 3% available chlorine based on the weight of the dry cellulose nitrate, removing said solution, washing the cellulose nitrate, and repeating said last three steps to further remove color and tree the cellulose nitrate of ammonia.

14. Process of treating cellulose nitrate comprising subjecting the cellulose nitrate in the undissolved state to the action of an ammonia solution to reduce the viscosity, treating the same with an aqueous solution containing available chlorine to terminate the viscosity reducing action or the ammonia, removing said solution contain ing available chlorine, boiling the cellulose nitrate in water, and repeating said treatment with chlorine solution and said boiling with water.

NICHOLAS BACON. 

